Background: South Sudan, the world's youngest nation, has been trapped in a cycle of conflict, political instability, and institutional decay since independence in 2011. Despite substantial oil wealth, the country ranks at the bottom of global development indices, reflecting systemic governance failure.

Objective: This study examined the relationship between leadership, legitimacy, and service delivery in South Sudan's fragile state context. It explored how leadership practices shape institutional legitimacy and public trust, and identified mechanisms through which leadership deficits undermine service delivery.

Methods: Drawing on neo-patrimonialism, state-in-society theory, and fragile state theory, the study employed a qualitative case study design with 25 participants comprising policymakers, civil society leaders, service delivery practitioners, and local administrators. Data were collected through semi-structured interviews and document analysis, and analysed using thematic analysis.

Findings: Leadership practices shape institutional legitimacy through personalized authority, ethnic patronage, and weak institutional culture, creating a self-reinforcing cycle of delegitimization. Leadership deficits undermine service delivery through resource diversion, political interference, fragmented service delivery, and erosion of public trust. Participants proposed institutional reforms including strengthening oversight institutions, decentralization, professionalising the civil service, and integrating traditional governance structures.

Conclusion: South Sudan's governance crisis is produced and sustained by leadership practices that prioritize elite accumulation over public welfare. Breaking the destructive cycle requires comprehensive reforms addressing neo-patrimonial practices, rebuilding institutional legitimacy, and restoring state capacity to deliver services.